1. Field of the Invention
The present invention relates to an apparatus and process for making a transfer sheet used in an injection molded printed-wiring board that is produced by a transfer method.
2. Related Art
A process for obtaining injection molded printed-wiring boards by a transfer method, is know in the art which uses a transfer sheet comprising laminated circuit units formed by laminate-printing using electroconductive and insulating pastes [Japanese Patent Kokai (Laid-Open) No. 63-219189]. A process using transfer sheet in which are provided a peeling layer comprising a material corroded with a solvent for electroconductive ink, a protective layer and printed circuit comprising an electroconductive ink is also known [Japanese Patent Kokai (Laid-Open) No. 63-257293]. Furthermore, also known is a process which uses the vapor deposition method, comprising a method of obtaining a metallic pattern printed board by providing a metallic film on an electroconductive primary film by plating, forming a pattern with a masking material, then further depositing a metallic film thereto to obtain a metallic pattern and then etching it [Japanese Patent Kokai (Laid-Open) No. 63-274194].
However, these transfer sheets suffer from various problems because either an electroconductive paste or a metallic conductor obtained by vapor deposition is used as a circuit conductor. When electroconductive paste is used, a circuit can be conveniently made by printing and drying, but it has a higher conductor resistance than metallic copper and the resulting circuit is unreliable. Furthermore, in the case of obtaining a copper circuit by vapor deposition, although the conductor resistance has no problems, it is difficult to obtain the conductor thickness necessary for printed-wiring boards. Furthermore, the resulting metal surface is flat, preventing the anchoring effect from being exhibited when adhering a substrate thereto and resulting in inferior adhesion to the substrate. A further problem is a very high cost. On the other hand, the copper foil generally used for printed-wiring boards has a high electrical reliability and results in a reliable circuit and has a roughened surface such that sufficient adhesion can be expected. However, in order to make a transfer sheet using this copper foil, it is necessary to laminate a carrier film and copper foil to each other with a suitable adhesive to form a laminate sheet.
The above adhesive is required to have an adhesive activity high enough to inhibit damage such as shifting or peeling of the circuit which may occur during production of the circuit pattern or injection molding and releasing power for easy peeling of the carrier film after molding which are contrary to each other in nature.
It is very difficult for general adhesives to have both of these properties. A sufficient solution has not yet been made in this respect.